1. Field of the Invention
The present invention relates to a semiconductor device and more particularly, to a semiconductor device including a MOS transistor that can adjust a substrate voltage.
2. Description of Related Art
In recent years, in semiconductor devices, a threshold voltage of a MOS transistor decreases in order to increase a switching speed and decrease power consumption. For example, in a dynamic random access memory (DRAM) that is an example of a representative semiconductor device, an operation voltage decreases to about 1 V. As a result, the threshold voltage of the MOS transistor also decreases to about 0 V.
Meanwhile, the threshold voltage of the MOS transistor is inevitably varied due to a process condition or a position on a wafer. As such, when the threshold voltage decreases, the variation in the threshold voltage particularly causes a problem in a circuit where a high sensitive operation is needed, for example, a sense amplifier that amplifies a small potential difference. Japanese Patent Application Laid-Open (JP-A) No. 2008-59680 discloses a method of controlling a substrate voltage of a MOS transistor to compensate for a variation in a threshold voltage.
However, in a recent minute transistor, since a substrate effect coefficient of the MOS transistor is small, the amount of the threshold voltage that can be adjusted by the substrate voltage is small. For this reason, if the substrate voltage is continuously varied to maintain the threshold voltage at a designed value, a variation width of a substrate potential may extraordinarily increase. This may vary a characteristic of another transistor whose threshold voltage does not need to be adjusted.
For example, when the MOS transistor whose threshold voltage needs to be adjusted is an N-channel MOS transistor constituting the sense amplifier, a characteristic of the MOS transistor constituting a memory cell may be deteriorated. Specifically, if the substrate voltage excessively increases, a charge of a memory cell capacitor is lost due to a subthreshold leak. In contrast, if the substrate voltage excessively decreases, the charge of the memory cell capacitor is lost due to a junction leak of a substrate with respect to a diffusion layer. Accordingly, the substrate voltage needs to be adjusted in a range of upper and lower limits not causing the leaks to increase.